Coordination effects on the binding of late 3d single metal species to cyanographene

Anchoring single metal atoms on suitable substrates is a convenient route towards materials with unique electronic and magnetic properties exploitable in a wide range of applications including sensors, data storage, and single atom catalysis (SAC). Among a large portfolio of available substrates, carbon-based materials derived from graphene and its derivatives have received growing concern due to their high affinity to metals combined with biocompatibility, low toxicity, and accessibility. Cyanographene (GCN) as highly functionalized graphene containing homogeneously distributed nitrile groups perpendicular to the surface offers exceptionally favourable arrangement for anchoring metal atoms enabling efficient charge exchange between the metal and the substrate. However, the binding characteristics of metal species can be significantly affected by the coordination effects. Here we employed density functional theory (DFT) calculations to analyse the role of coordination in the binding of late 3d cations (Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Cu+, and Zn2+) to GCN in aqueous solutions. The inspection of several plausible coordination types revealed the most favourable arrangements. Among the studied species, copper cations were found to be the most tightly bonded to GCN, which was also confirmed by the X-ray photoelectron spectroscopy (XPS), atomic absorption spectroscopy (AAS), and isothermal titration calorimetry (ITC) measurements. In general, the inclusion of coordination effects significantly reduced the binding affinities predicted by implicit solvation models. Clearly, to build-up reliable models of SAC architectures in the environments enabling the formation of a coordination sphere, such effects need to be properly taken into account.


Table S1
Selected aqua-complexes of late 3d transition metal cations with given ground state electronic configuration, the most common multiplicity and symmetry.

Table S2
Bonding characteristics of aqua-complexes of 3d transition metals bonded to GCN and acetonitrile (ACN) in water: symmetry (Symmetry) and multiplicity (M) of aqua-complex, local symmetry of the aqua-complex bonded to GCN/ACN (*Symmetry), the natural charge of Me in aqua-complex (q i ) and that of the metal atom in an aqua-complex bonded to GCN (q f ), the difference of Mulliken charges (∆q), N-Me bond length in Å (d), the binding energy (in kcal/mol) computed at the PBE0/def2-TZVP level (Model A, frozen-sheet approach).                        Additional experimental data

Determination of functionalization degree of GCN:
In order to experimentally determine thermodynamic characteristics of the Cu 2+ coordination on GCN, functionalization degree of GCN needed to be determined to know what quantity of CN groups is in certain mass of GCN. This was achieved through the same rational used in the work of Bakandritsos et al. in the reference. 2 For this, we analyzed the graphene acid (GA) sample synthesized from the same batch of GCN used in this study. Figure S1: Survey XPS analyses of GCN and graphene acid (GA) synthesized thereof with determined atomic compositions.
According to XPS analysis ( Figure S3a) in GCN there is 12.9 at. % of N. Residual 3.8 at. % of N found in GA ( Figure S1) is considered as contaminant not related to CN groups. Therefore. there is 9.1 at. % of CN groups in our GCN material. The formula representing the composition of our GCN therefore is: ( ) 9.1 3.8 71.9 4.3 1.4 The molecular weight of this hypothetical unit therefore is: from which 236.6 g·mol -1 corresponds to CN groups alone. Therefore, the mass fraction of CN groups in our GCN is:

Monitoring of GCN-Me samples washing
The monitoring of was done to observe the trend how the residual unbonded metal ions were released from the material during their washings. The volume of water used for each washing step was the same as used for the immobilization of the metal. The time of redispersion of the filtered material was not fixed, but usually within several minutes. Considering the final adsorbed amounts of metals on the GCN, the amounts of metals in all the analyzed filtrates were overall in all cases lower than the initial amounts due to due to adventitious dilution of the filtrates and possible metal sorption on the filtration apparatus.

Isothermal titration calorimetry
The ITC experiments were performed at the temperature of 25 °C. Overall, three measurements were carried out, in which 800 µL of GCN dispersion (twice with the concentration of 3.125 mg/mL which is equivalent to 22.84 mM in terms of CN groups, once as 7.6mM CN solution) was titrated with Cu(NO 3 ) 2 solution by injecting 60 x 3 µL of 50mM solution, 60 x 3 µL of 37.5mM solution, and 50 x 4 µL of 10mM solution, respectively. The time interval between each injection was 5 minutes.
In all three experiments (Figure S5 a, c, e), initial injections of Cu 2+ into GCN resulted in a sudden exothermic response that could be attributed to the interactions of Cu ions with highly reactive sites such as radical defects and vacancies in the GCN structure. With more injections, the response changed to endothermic, which after peaking gradually flipped to a slightly exothermic regime upon continued injection of Cu 2+ . Since the endothermic response occurred in a shorter timescale when higher concentration of Cu 2+ solution was used for the titration (cf. panels a and c in Figure S6), the endothermic response was linked to the immobilization of copper ions on the CN groups of GCN. After the saturation of the CN groups, the slightly exothermic processes after each injection that occurred consistently till the end of the measurement was attributed to Cu 2+ ions diluting in the mixture.
The obtained data were fitted using the independent model (panels b, d, and f in Figure S5) as implemented in NanoAnalyze Data Analysis software ver. 3.12.00. Although the multiple sites model fitted the initial part of the enthalpic data better, it provided enthalpy values with extremely high standard deviations indicating the numerical instability of the model, unlike the independent model. The discrepancy between the independent fit and the initial part of the data was caused by the fact that the exothermic process took place in the initial phase of injecting.
To check the sensitivity of the independent model, we performed a series of fits excluding up to four initial recorded points. The derived thermodynamic quantities for experiments 1-3 as a function of the number of excluded points are listed in Tables S28-S30, respectively. The mean values of the thermodynamic quantities with their standard deviations are listed in Table S31 for each experiment along with values determined from all the determined values in the last row of the table. Corrected heated rate data measured in the first (a), second (c), and third experiment (e), and plotted enthalpy/molar ratio dependencies generated from the heat rate data of the first (b), second (d), and third experiment (f) and fitted using the independent model, which afforded the thermodynamic quantities that can be found in Tables S28-S31.